Amplifier tube and circuit



J., 31, 1939. t W. J. OBRIEN 2,345,368

AMPLIFIER TUBE AND CIRCUITy Filed De'c. 27, 1934 INVENTOR. WIL IAM J. O'BREN ATTORNEY.

Patented Jan. 31, 1939 UNITED STATES PATENT FFCE AMPLIFIER TUBE ANI)y CIRCUIT of Delaware Application December 27, 1934, Serial No. 759,327

2 Claims.

An object of this invention is to provide an amplifier tube and circuit which is adapted to amplify currents which have a Wide range of frequency, which currents may be of either audio or radio frequency, the amplifier being arranged to amplify currents of the different applied frequencies substantially equally.

A further object of the invention is to provide an electronic discharge device which has a plug; rality of grids one of which serves as a screen grid and is so arranged that its voltage varies in phase with that of the signal control grid.

A still further object of the invention is to provide a vacuum tube having a plurality of grids including a signal control grid and a fourth grid located between the cathode and thev signal control grid, this grid being connected and arranged to keep the current flowing to the screen grid from increasing excessively when the screen voltage is made positive by a substantial amount.

Other objects of the invention will become apparent to those skilled in the art as the description thereof proceeds. For a better understanding of the invention, however, reference is made to the following specification taken in connection with the attached drawing which shows one embodiment of the invention v In the drawing,

Figure l is a schematic circuit diagram of an amplifier arrangement constructed according to the invention and,

Figure 2 is a schematic diagram showing the arrangement of several electrodes within the tube.

Referring rst to Figure 2, the vacuum tube 2i? comprises a cathode C which is heated to electron emitting temperature by a heater or filament 2i and a first grid i which, as shown, is connected to the cathode within the tube but this connection may be by a lead ll outside the tube, as shown in Figure l. The second grid 2 is shown connected to the fourth grid 4 within the tube and between these grids is located the signal control grid 3. The tube also has the usual output l, electrode or plate 6, and, as shown, has an auxiliary grid 5 located between the plate and grid l?. Suitable lead Wires are connected within the tube to the several elements thereof and pass through the tube envelope to the exterior thereof.

As shown in Figure l, two tubes of the type described, are connected together to form an aperiodic amplifier, an input circuit comprising resistor 8 and grid bias battery I3 being connected between grid 3 and the cathode C of the first stage. Any suitable source of voltage of variable frequency may be connected to the terminals of resistor 8. As shown in the drawing such source is a photoelectric cell 9 Whose terminals are connected to the terminals of resistor 8. The currents generated by the cell il may be of audio or radio frequency type, depending on the use to which the amplifier is to be put, and while I have shown a photo-cell as a source of variable frequency currents it will be understood that any other signal voltage source of either audio or radio frequency type may be used instead. As shown, the positive terminal of the plate battery l5 is connected to the plate through a high resistance 'it and grids 2 and 4 connected to an inter- `mediate point of the battery through a .resistor l2. Also the grid 5 is shown connected to another intermediate pont of the battery of higher potential but it will be understood that this con* nection may be omitted and the grid 5 directly connected to the cathode so that this lgrid operates as a suppressor grid. As shown, the battery connections to the several elements of the second tube are the same as those of the iirst and therefore need not be further described. Also as shown, the plate of the first stage is coupled to the control grid 3 of the second stage through a condenser 'l and the amplied voltage is obtained across the output terminals iti at the ends of resistor I@ of the second stage. These output terminals, it will be understood, may be connected to any desired utilization device in any known manner. The capacity between the input terminals of the first stage is designated by the condenser M which is dotted as this is inherent capacity and not a physical condenser.

It is known that an amplifier which must give U fairly uniform amplification for signals covering a wide range of frequencies beginning at the lower end of the audio range and extending into the radio frequency range has a limitation in the upper range due to the capacity of the input l and output circuits of the several stages. In the usual amplifier tube when a signal voltage is ap-l plied to the grid, the grid and plate voltages are of opposite phase and therefore the voltage between these elements is equal to the gain plus one times that which exists between grid and ground or cathode. The signal current from grid to plate through the capacity between these elements will be increased by an amount equal to the voltage increase. This is in fact an increase of input capacity by an amount equal to the gain times the grid to plate capacity. This effect has been substantially eliminated by the use of screen grid type tubes and these tubes perform much better than triodes for amplifying a Wide range of frequencies.

I have found that a screen or auxiliary grid which is actuated by a potential which is in phase with that of the control grid will effectively de crease to a considerable extent the capacity of the input circuit for the same reason that the 180 difference in potential between control grid and plate effects an increase. rIhis result may be accomplished as shown in Fig. 1 by providing at least one grid such as the grid 2 adjacent the grid 3 so that it has a negative trans-conductance with respect to the latter. In my device, grid I substantially limits the total current emitted from the cathode and prevents the voltage variation of grid 2 from substantially changing this current value. As grid I limits the total emission current from the cathode, the current to plate 6 plus the current to grid 2 must equal a constant value, as they make up the total cathode current. Therefore any increase in current to plate 6 from whatever cause which does not afect the total cathode current must be accompanied by a corresponding decrease in the current to grid 2. Due to the relative position of the grids and plate in the tube a more positive potential applied to grid 3 produces an increase in the velocity of the electrons toward plate 6 in the region of grid 3 thereby allowing more of them to pass beyond this region and travel to the plate. These electrons otherwise would have passed to grid 2, and therefore the number passing to this grid has been decreased. When control grid 3 is made more positive by the applied signal voltage the current through resistor I0 increases and the voltage of plate 6 decreases; but at the same time, due to the relative positions of the grids, the current flowing to grid 2 decreases so that the voltage of point I I and grid 2 likewise increase or, in other words, the trans-conductance of grid 2 with respect to grid 3 is negative so that Athe voltage variations of grids 2 and 3 occur in phase whereas those of grid 3 and plate 6 are out of phase. Grid 5 acts as a shield grid between grid 4 and plate 5 and prevents an effective increase of capacity between them due to the voltage variation of the plate occasioned by the signal voltage. Grid 5 may be operated either as a suppressor or a screen grid, its use depending on the negative transconductance of grids 2 and 4 with reference to grid 3.

In the circuit shown in Figure 1, the resistors l2 control the gain between grid 3 and grid 2. When this gain is made unity by choice of a proper value of resistor I2, grid 3 and grids 2 and Il will upon application of a signal voltage, fluctuate in potential equally and in phase with each other so that they will remain at the same radio frequency potential and will, therefore, give the result of an infinite input impedance. If resistor I2 is so proportioned as to make the gain greater than this amount the result will be that the input will be equivalent to a negative capacity in series with a negative resistance.

This negative capacity transferred to the input circuit will balance out the positive capacity I4 transferred thereto by the inherent capacity between the circuit of plate 6 and that of grid 3 including the capacity between these elements within the tube. It will be understood that any undesired capacity I4 in the input circuit causes the amplification to fall off fairly rapidly as the applied frequency is increased but by means of this invention this capacity may be neutralized or compensated for by the negative capacity provided by grid 3 in connection with grids 2 and 4. By making the gain of that portion of the tube including grids 3 and 2 greater than unity as by a proper choice of the value of resistor I2, the inherent capacity I4 may be completely compensated for and with the result that all the frequencies within the range to be amplified are amplified substantially equally, The increased amplification of the high frequencies over the usual triode amplifier as provided by the rst stage is effected in substantially the same manner in the second stage so that the effect of the two stages is cumulative and it will be understood that further similar amplification stages may be added if desired. I have found the following values to give satisfactory results in the arrangement described; resistor III may have a value substantially equal to the plate resistance of the tube, resistor I2 has a value of 4,000 ohms when the negative transconductance of grid 2 has a value of 300 micromhos, resistor 8:500 megohms.

While I have shown my invention in a single embodiment thereof it is to be understood that it it not restricted to the specific embodiment shown and is limited only by the scope of the following claims and the prior art.

Having described my invention what I claim as novel and desire to secure by Letters Patent is:

1. An aperiodic amplifier for amplifying currents covering a wide band of frequencies, comprising the combination of an electronic tube having a cathode, a signal control grid, a plate, a screen grid located between said cathode and control grid, third and fourth grids located between said signal control grid and plate, an input circuit connecting said signal control grid and cathode including means for providing a negative bias potential to said control grid, a source of steady potential having its negative terminal connected to said cathode, a circuit including an impedance connecting the positive terminal of said potential source and said plate, means for maintaining said fourth grid at a steady positive potential which is less than that of said potential source, and a resistor having one end connected to an intermediate point of said potential source and its opposite end to said screen grid and also to said third grid.

2. An aperiodic amplifier according to claim l, wherein the electronic tube is provided with an auxiliary grid positioned between the screen grid and the cathode, said auxiliary grid being at cathode potential.

WILLIAM JOSEPH OBRIEN. 

